Arrow detection system employing a sweeping laser beam

ABSTRACT

An automated archery range including a firing line from which arrows are shot toward a penetrable target, a backstop behind the target for stopping arrows, an arrow collector beneath the backstop, an arrow return conveyor for receiving arrows from the collector and returning them toward the firing line, and an arrow storage quiver adjacent the firing line for receiving arrows from the conveyor and storing the arrows in upright positions readily accessible to the archer.

United States Patent David P. J. McLeod;

Patrick J. Murphy, both of Muskegon, Mich.

Feb. 14, 1969 Nov. 9, 1971 Bnmswick Corporation Inventors App]. No. Filed Patented Assignee ARROW DETECTION SYSTEM EMPLOYING A SWEEPING LASER BEAM 13 Claims, 6 Drawing Figs.

US. Cl 250/222, 273/102.2, 250/216, 250/236 Int. Cl G06m 7/00, 601p 3/68 Field of Search 356/203,

[56] References Cited UNITED STATES PATENTS 2,971,695 2/ 1961 Sick 250/224 X 3,328,523 6/1967 Treseder et l78/7.6 X 3,401,937 9/1968 Rockwood et a1... l78/7.6 3,436,540 4/1969 Lamorlette 250/222 X 3,500,063 3/1970 Reno 250/222 X Primary Examiner-Walter Stolwein Attorney-Hofgren, Wegren, Allen, Stellman & McCord ABSTRACT: An automated archery range including a firing line from which arrows are shot toward a penetrable target, a backstop behind the target for stopping arrows, an arrow collector beneath the backstop, an arrow return conveyor for receiving arrows from the collector and returning them toward the firing line, and an arrow storage quiver adjacent the firing line for receiving arrows from the conveyor and storing the arrows in upright positions readily accessible to the archer.

PAIENTEDunv 9197i SHEET 3 0F 3 m lm U 1 {ill};

BACKGROUND OF THE INVENTION In the recent past, efforts have been directed toward provision of automated archery lanes, particularly for indoor use, involving a target remote from a firing line constructed in a way such that arrows do not remain impaled in the target but fall free for collection to be returned automatically to the archer at the firing line. In preferred systems, the target is constructed to be penetrable so that neither the target nor the arrow is substantially damaged by target penetration. Behind the target there is a suitable backstop which is usually yieldable in a way to absorb the energy of the arrow so that the latter stops and falls downwardly to an appropriate means for directing the arrow to a return conveyor. The return conveyor has preferably been in the form of a conveyor belt means which delivers the arrows to a container adjacent the firing line and accessible to the archer so that he merely has to removed an arrow from the storage container, fire at the target and await return of the arrow to the storage container. The return may be accomplished in a matter of a few seconds so that the archer may effectively practice his sport with only one or two arrows, if desired. Because the arrows do not remain impaled in a target for inspection by the archer, it has been contemplated in such systems that there would be a sensing apparatus for determining the location of the arrow hit in the target and controlling an indicating means adjacent the firing line for showing the archer where the arrow struck the target.

SUMMARY OF THE INVENTION The invention relates to a system for detecting the point of passage of a projectile along a path and in the exemplary embodiment, a pair of reflectors are located along one side of the path, which reflectors are formed of a retrorefleetive material that has the property of reflecting a beam of light back towards its source without regard to the angle of incidence of the light beam on the reflector. On the opposite side of the path there is provided a source of light in the form of a double ended laser which is flanked by a pair of mirrors which rotate at high speed to direct light beams emanating from both ends of the laser toward respective ones of the reflectors. Interposed between the ends of the laser and the flanking rotary mirrors are inclined apertured mirrors, the apertures being provided to permit the laser beam to pass from the ends of the laser to the adjacent mirrors which receive the beam of light passed from the laser ends to the rotary mirrors to the reflectors, and back to the rotary mirrors, and reflect the same towards respective photocells. Whenever the beams of light are broken by a projectile, the temporary lack of illumination of the photocells provides a signal that indicates that, at a particular angular position of the respective rotary mirror, the presence of a projectile was sensed. This indication is, therefore, an indication of the position of the projectile within the scanned path.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of an archery installation, looking down a pair of lanes toward a pair of targets from a position adjacent an arrow quiver;

FIG. 2 is a perspective view partly broken away, illustrating a housing for the targets, backstops and arrow collectors for delivering arrows to a return conveyor;

FIG. 3 is a perspective view of the targets and scanning system looking from the rear of the housing with parts broken away;

FIG. 4 is a plan view of the scanning system;

FIG. 5 is a fragmentary vertical section of a housing for a light source, rotary mirrors, inclined mirrors and photocells; and

FIG. 6 is a fragmentary vertical section of the housing for the mirrors and light source and photocells.

DETAILED DESCRIPTION Referring now particularly to FIGS. 1 and 2, there is illustrated a substantially complete installation for two automated archery lanes side by side in which the entire apparatus is disassemblable and/or portable to permit removal from the floor surface utilized in order to leave it free for use for other purposes. As shown, the installation includes an arrow storage quiver 10 adjacent a firing line 11 adapted to serve two adjacent lanes 12 and 13 in that it is constructed to hold arrows as at 15 in upright positions, either point up or fletching up, conveniently disposed for easy access by archers on both lanes. Targets for both lanes are provided in housing 18 located remotely from the firing line 11 and supported on wheels which facilitate adjustment of the housing toward and away from the firing line to permit adjustment in the length of the range.

In order to provide a target on each lane, the wall of the housing 18 facing the firing line is formed with a pair of large rectangular openings as at 19 and 20 and each aperture is closed'by a penetrable screen 22 adapted to carry a target pat tern as at 24 and constructed in a manner to permit arrows to pass through the screen without substantial damage to the screen or to the arrows. In a preferred fonn, the penetrable screen 22 comprises a plurality of vertically disposed closely adjacent flexible strands anchored at the top and bottom to the housing 18. For example, the strands may be one-eighth inch natural rubber or vinyl strands which provide both a suitable surface for the target and also long life with repeated arrow penetration. The target 24 may be painted on the screen 22 or may be an image projected onto the screen. The latter form has the advantage that the form of the target may be readily changed as desired.

At the rear of the housing l8, behind the target screens 22 there are suitable backstop means as at 28. As illustrated, each of the backstop means 28 is in the form of a free hanging net disposed in front of a fixed net in a manner such that the energy of the arrows fired through the target screens is absorbed by the backstop means in a manner to stop the arrow without damage to the arrow or the backstop, as a result of which the arrow falls downwardly for return toward the firing line. Other backstop means may be utilized and one acceptable form includes the use of a free hanging bed of many strands of flexible material such as plastic tubing in sufficient numbers to provide a relatively thick barrier to the passage of arrows and yet have sufficient flexibility to absorb the energy of the arrows without rebound of the arrows.

The sidewalls and the top of the housing 18 may be suitably covered with appropriate material as at 29.

From the backstop means 28, the arrows fall downwardly toward suitable means for directing arrows from both lanes toward a central common return conveyor. As illustrated, the arrow gathering or collecting means in each lane comprises an endless conveyor as at 30 having a width substantially equal to the distance between the screen 22 and the backstop 28 and disposed to travel from the outer edge of the housing toward the center of the housing as represented by indicating arrows 32. The cross conveyor belts 30 are each arranged to pass about a pair of long support and drive rollers as at 34 and 35 on housing 18 at least one of which is arranged to be rotated by suitable drive means as at 36. Arrows fall from the backstop means 28 to the cross conveyors 30 as illustrated at 38, for example. While the arrows shown are disposed with the pointed ends leading for return to the firing line, some of the arrows fall from the backstop with the fletched end disposed toward the firing line, and the arrow return system is adapted to handle either arrangement easily. In practice, substantially more arrows return point first than fletching first.

The cross conveyors 30 deliver fallen arrows to a centrally disposed common arrow return conveyor 40 including an endless conveyor belt 41 supported adjacent the firing line on an idler roller associated with the quiver l0, and adjacent the housing 18 by a drive pulley on a motor 43. Intermediate the idler roller and the drive pulley, the long upper and lower reaches of the arrow return conveyor are supported by a channel structure 45. The arrangement is such that the conveyor belt 41 is driven at a relatively rapid rate so that arrows are returned in a matter of a few seconds and are thrown into the quiver 10 with sufficient force to reach the storage positions 15.

In order to detect the position of an arrow relative to the target 24 as the arrow passes through the penetrable screen 22, there is an arrow detection system in the housing 18 including a housing 48 located centrally between the apertures 19 and 20 and including a light and optical scanning system for sweeping two beams of light across each target area to provide two angular measurements in the forms of angular coordinates for indicating the position of the arrow. For example, as seen in FIG. 2, one beam of light is swept across the righthand target area from an effective starting position represented by line 49 to an effective finish position represented by line 50. Each light beam is directed toward a reflective strip on housing post 51. Preferably the detection system is used for purposes of controlling an indicating means associated with the quiver l and including an indicator face for each lane as at 52 and 53 bearing an image as at 54 simulating the target 24. In a preferred form the indicator includes an indicator light movable about the indicating face and controlled by the arrow detection system.

The scanning system can best be understood with reference to FlGS. 3-6. As viewed in FIG. 3, the scanning unit, generally designated 100, is interposed between the targets 24 of the adjacent lanes 12 and 13. As will be seen hereinafter, the scanning unit 100 includes an upper scanner 102 and a lower scanner 104 and both scanners are operative to scan both of the lanes 12 and 13.

On the outboard side of the lane 13 and opposite from the scanning unit 100, there is located a pair of vertically oriented retroreflective tapes 106 and at the upper extremity of each of the tapes 106 there is located a start photocell 108.

Similarly, at the outboard side of the lane 12 and opposite the scanning unit 100, a pair of similar tapes 106 are located and at the lower extremity of each tape 106 there is provided a start photocell 108.

The retroreflective tapes 106 for each of the lanes 12 and 13 are horizontally spaced and the lower scanner 104 is adapted to project a rotating beam of light at the rearrnost tape 106 associated with the lane 13 as well as the forwardmost tape 106 associated with the lane 12. The upper scanner 102 is adapted to project a rotating beam of light at the forwardmost tape 106 associated with the lane 13 and the rearmost tape 106 associated with the lane 12. Stated another way, the beams of light provided by both the scanners 102 and 104 are in a generally vertical plane which is slightly askew to a position normal to the length of the lanes 12 and 13 with the plane of the beam of the upper scanner 102 being askew in the direction opposite from the direction in which the plane containing the beam of light from the lower scanner 104 is askew.

The foregoing relation is illustrated in FIG. 4 wherein the beam of light generated by the upper scanner 102 is designated 1 while the beam of light generated by the lower scanner 104 is designated 112.

The retroreflective tapes 106 are commercially known as 3M Brand photoelectric scanning tape sold by Minnesota Mining and Manufacturing Company and have the property of returning the light directly to its origin regardless of the striking angle. As a result, when a beam of light strikes the retroreflective tape 106, the beam of light will be reflected back towards its source without regard to the angle that the beam of light initially impinged upon the tape. The retroreflective tapes 106 are also of a sufficient length so that a moving beam of light from the upper scanner 102 will be able to completely sweep across the targets 24 and be returned to the upper scanner 102 provided that the beam has not been broken by a projectile such as an arrow. Similarly, the retrorefiective tapes 106 associated with the lower scanner 104 have a length to provide the same capability with respect to the beam of light provided by the lower scanner 104. With reference to FIG. 3, the beams of light from the scanners 102 and 104 rotate in a counterclockwise direction.

Turning now to FIGS. 5 and 6, the construction of the scanning unit may be seen. A vertically elongated housing is provided and within the same at about the vertical midpoint thereof, there is mounted a double ended laser 122 of conventional construction. Also located within the housing 120 is a power source 124 for the laser 122.

The double ended laser 122 may be of the type obtainable from Optics Technology, Inc. of Palo Alto, Calif. and has the ability to provide two coaxial light beams, one from each end, of very narrow size. One such light beam is utilized by the upper scanner 102 while the other light beam is utilized by the lower scanner 104.

The use of the double ended laser 122 allows the scanning of two areas with but a single energy source. Furthermore, the laser is capable of producing a light beam having a diameter significantly smaller than that of an arrow and with a high energy level so as to maximize system activity.

The manner in which the light beam from the laser 122 is utilized will now be described in conjunction with the lower scanner 104 with the understanding that the construction of the upper scanner 102 is identical. A mounting plate 126 within the housing 120 supports, near its lower end, a high speed synchronous motor 128 having a rotary output shaft 130 to which a mirror 132 is amxed. The mirror 132 is externally silvered on both of its sides and, when driven by the motor 128, will rotate to deflect the beam of light from the laser 122 in a sweeping path just behind the target 24 (FlGS. 3 and 4) to the retroreflective tapes 106 with which it is associated on both of the lanes 12 and 13. In the exemplary embodiment, the synchronous motor 128 operates at 3600 r.p.m. and the silvering of both sides of the mirror results in a scan rate of 7200 scans per minute.

The laser 122 is located directly above the mirror 132 and arranged so that the beam of light emanating from one end thereof is focused on the mirror to intersect the axis of rotation thereof. interposed between the laser 122 and the mirror 132 is an inclined, totally silvered mirror 134 having an aperture 136 therein through which the beam of light from the laser 122 may pass to the mirror 132. The mirror 134 is located at an angle so that a beam of light reflected by the mirror 132 towards the mirror 134 and not passing through the aperture 136 will be directed toward a lens 138 mounted on the plate 126 which then focuses the reflected beam of light on a stop photocell 140.

Finally, the housing 102 is provided with windows 142 adjacent to the mirror 132 through which the laser beam may be reflected by the mirror 132 externally of the housing to the tape 106.

From the foregoing description, it will be appreciated that normally the photocell 140 will be illuminated whenever the beam of light 112 provided by the lower scanner 104 is sweeping across the confines of the target 24. This is due to the fact that as the mirror 132 rotates to sweep the beam of light 112, and the latter impinges upon the retroreflective tape 106, the light beam will then be reflected back to the mirror 132 and, in turn, to the mirror 134 to be focused by the lens 138 on the photocell 140. The only time during the scan of the target 14 that the photocell 140 will not be illuminated is when the beam of light 112 is broken. This will occur when a projectile is within the scanned area and breaks the beam of light 112. Accordingly, the photocell 140 provides an indication of the time when the beam of light is broken by a projectile.

In order to insure that the beam of light is broken by a projectile, it is necessary that the angular velocity of the sweeping beam of light be such that it will completely scan the target area 14 before projectiles can pass through the plane in which the light beam rotates. When, as is the case with the exemplary embodiment, the scanning system is intended for use in detecting arrows, the above-mentioned scan rate is sufficient to detect even the shortest arrows now used, even if shot at the target at the highest velocity obtainable with current bows. Projectiles having a shorter length than arrows and/or fired at the target at higher velocity than a high velocity arrow, are capable of detection using the principles of the invention. For such shorter projectiles and/or higher velocity projectiles, it will, of course, be necessary to increase the scan rate and it may also be desirable to use extremely high quality photocells that are extremely sensitive to rapid changes in illumination as when the beam of light is broken by the projectile.

Returning to FIG. 3, it will be observed that the location of the start photocell 108 is such that for the direction of rotation of each light beam 112 or 110, the photocell 108 will be briefly illuminated at the beginning of each sweep of the beam of light across the target. As a result, the photocells 108 provide a signal indicating when in time a scanning cycle has been initiated. Because synchronous motors 128 are used to drive the beam of light through its scan, the rate of the scan is constant. It can be determined when the scan was initiated and when in the scan the beam was interrupted by interrogating the photocells 108 and 104 and thus, the angular position of the beam of light at the time it was interrupted can be determined to thereby provide information relative to one coordinate of the point of impact of the projectile on the target 24.

As mentioned previously, the upper scanner 102 works in an identical manner but provides information relative to a second coordinate of the point of impact. By means of a computer that utilizes information from the photocells 108 and 140 for both the upper and lower scanners 102 and 104 on one lane, two coordinates of the point of impact of a projectile are determined, and thus, the exact point of impact can be determined. An appropriate computer circuit suitable for use with the scanning system of the instant invention is disclosed in U.S. Pat. application Ser. No. 799,451, filed Feb. 14, 1969 and assigned to the same assignee as the instant invention, and reference may be had thereto for an understanding of the computer utilized.

What is claimed is:

1. In a system for detecting the position of a projectile in either of two adjacent paths; a pair of reflectors disposed at opposite sides of the adjacent paths; an optical system between the paths including a light source, a rotatable mirror in the path of light emanating from the source disposed to sweep a beam of light across the projectile paths along the reflectors, and means for rotating the mirror; a pair of start photocells, one adjacent each of a respective one of said reflectors in the path of the sweep of the light beam and responsive to the passage of the beam to denote the beginning of the sweep across the projectile path; a mirror between the light source and the rotatable mirror constructed for passing light from the source to the rotatable mirror and having a reflective surface for receiving light from the rotatable mirror reflected from either of said reflectors; and an interrupt photocell disposed to receive light from the inclined mirror and responsive to the interruption of reflected light and the passage of a projectile along either of said paths to denote the angular position of the projectile relative to the photocell adjacent the reflector.

2. A combination as defined in claim 1 including a condensing lens between the inclined mirror and the interrupt photocell for concentrating reflected light on the interrupt photocell.

3. A combination as defined in claim I wherein the inclined mirror is totally reflective and has a central aperture for passing light from the source to the rotatable mirror.

4. A combination as defined in claim 3 wherein the light source is a laser unit for directing a laser beam through the aperture in the inclined mirror.

5. The combination as defined in claim 1 wherein each of said reflectors comprises a strip of material adapted to reflect substantially all light back toward the source throughout a wide range of angles of incidence at which the light strikes strip.

6. In a system for detecting the position of a projectile in either of a pair of adjacent paths and an associated penetrable target, a pair of adjacent reflective strips along one side of each path adjacent the corresponding target, adapted to reflect light back in the direction from which it comes, a single light source, a pair of optical systems interposed between the paths, one associated with each reflective strip associated with each path and spaced about said light source, and each including a rotatable mirror in the path of light emanating from the source and disposed to sweep a beam of light across the projectile paths and along the associated reflective strips, a start photocell at one end of each reflective strip in the path of the sweep of the associated light beam responsive to the beam to denote the beginning of the sweep of the beam, an inclined mirror between said light source and the associated rotatable mirror constructed to pass light from the source to the rotatable mirror and having a reflective surface for receiving light from the rotatable mirror reflected from the reflective strips, and a stop photocell associated with each inclined mirror to receive light therefrom and responsive to change in reflected light on passage of a projectile along either path to denote the angular position of the projectile relative to the associated start photocell.

7. A combination as defined in claim 6 wherein each inclined mirror is totally reflective and has an aperture for passing light from the light source to the associated rotatable mirror.

8. A combination as defined in claim 2 wherein the light source comprises a laser unit for directing laser beams out opposite ends thereof in opposite directions through the associated apertures toward the rotating mirrors.

9. In an apparatus for detecting the locations of arrows passing through a pair of adjacent penetrable targets, an optical housing disposed between the targets, a source of a highly collimated beam of light in the housing, a rotatable mirror in the housing disposed to sweep a beam of light from the source across each target as the mirror rotates, a reflective strip alongside each target opposed to the optical housing for receiving light swept across the target and reflecting the light back toward the rotatable mirror, a start photocell adjacent one end of each strip in the path of the light to denote the beginning of the sweep along each strip, an inclined mirror between the light source and the rotatable mirror for passing light from the source to the rotatable mirror and having a reflective surface to reflect light from the rotatable mirror reflected from each strip, and a stop photocell disposed to receive light from the inclined mirror and responsive to the change in light on passage of an arrow to denote the angular position of the arrow relative to the start photocell.

10. In an apparatus for detecting the locations of arrows passing through a pair of adjacent penetrable targets, an optical housing disposed between the targets, a source of light in the housing, a rotatable mirror in the housing disposed to sweep a beam of light from the source across each target as the mirror rotates, a reflective strip alongside each target opposed to the optical housing for receiving light swept across the target and reflecting the light back toward the rotatable mirror, a start photocell adjacent one end of each strip in the path of the light to denote the beginning of the sweep along each strip, an inclined mirror between the light source and the rotatable mirror for passing light from the source to the rotatable mirror and having a reflected surface to reflect light from the rotatable mirror reflected from each strip, a stop photocell disposed to receive light from the inclined mirror and responsive to the change in light on passage of an arrow to denote the angular position of the arrow relative to the start photocell, a

second source of light in the housing, a second rotatable milror in the housing disposed to sweep a beam of light from the second source across each target on rotation, a second reflective strip alongside each target adjacent the first recited strip for receiving light from the second source and reflecting the light back to the second rotatable minor, a start photocell adjacent one end of each second reflective strip in the path of light from the second sourc eto denote the beginning of the sweep along the strip, an inclined mirror between the second light source and the second rotatable mirror for passing light from the source to the rotatable mirror and having a reflective surface to reflect light from the rotatable mirror reflected from each second strip, and a second stop photocell disposed to receive light from the second inclined mirror and responsive to change in light on passage of an arrow to denote the angular position of the arrow relative to the start photocell.

11. A combination as defined in claim 5, wherein the rotatable minors each have two reflective surfaces to sweep two successive beams of light onfih revolution.

12. A combination as defined in claim 5, wherein the rotatable minors are disposed to sweep their beams in planes intersecting each other in the optical housing so that each mirror scans a forwardly disposed strip adjacent one target and a rearwardly disposed strip adjacent the other target.

13. A combination as defined in claim 7, wherein said light sources comprise a laser unit for directing laser beams coaxially out opposite ends thereof toward the rotatable mirrors. 

1. In a system for detecting the position of a projectile in either of two adjacent paths; a pair of reflectors disposed at opposite sides of the adjacent paths; an optical system between the paths including a light source, a rotatable mirror in the path of light emanating from the source disposed to sweep a beam of light across the projectile paths along the reflectors, and means for rotating the mirror; a pair of start photocells, one adjacent each of a respective one of said reflectors in the path of the sweep of the light beam and responsive to the passage of the beam to denote the beginning of the sweep across the projectile path; a mirror between the light source and the rotatable mirror constructed for passing light from the source to the rotatable mirror and having a reflective surface for receiving light from the rotatable mirror reflected from either of said reflectors; and an interrupt photocell disposed to receive light from the inclined mirror and responsive to the interruption of reflected light and the passage of a projectile along either of said paths to denote the angular position of the projectile relative to the photocell adjacent the reflector.
 2. A combination as defined in claim 1 including a condensing lens between the inclined mirror and the interrupt photocell for concentrating reflected light on the interrupt photocell.
 3. A combination as defined in claim 1 wherein the inclined mirror is totally reflective and has a central aperture for passing light from the source to the rotatable mirror.
 4. A combination as defined in claim 3 wherein the light source is a laser unit for directing a laser beam through the aperture in the inclined mirror.
 5. The combination as defined in claim 1 wherein each of said reflectors comprises a strip of material adapted to reflect substantially all light back toward the source throughout a wide range of angles of incidence at which the light strikes strip.
 6. In a system for detecting the position of a projectile in either of a pair of adjacent paths and an associated penetrable target, a pair of adjacent reflective strips along one side of each path adjacent the corresponding target, adapted to reflect light back in the direction from which it comes, a single light source, a pair of optical systems interposed between the paths, one associated with each reflective strip associated with each path and spaced about said light source, and each including a rotatable mirror in the path of light emanating from the source and disposed to sweep a beam of light across the projectile paths and along the associated reflective strips, a start photocell at one end of each reflective strip in the path of the sweep of the associated light beam responsive to the beam to denote the beginning of the sweep of the beam, an inclined mirror between said light source and the associated rotatable mirror constructed to pass light from the source to the rotatable mirror and having a reflective surface for receiving light from the rotatable mirror reflected from the reflective strips, and a stop photocell associated with each inclined mirror to receive light therefrom and responsive to change in reflected light on passage of a projectile along either path to denote the angular position of the projectile relative to the associated start photocell.
 7. A combination as defined in claim 6 wherein each inclined mirror is totally reflective and has an aperture for passing light from the light source to the associated rotatable mirror.
 8. A combination as defined in claim 2 wherein the light source comprises a laser unit for directing laser beams out opposite ends thereof in opposite directions through the associated apertures toward the rotating mirrors.
 9. In an apparatus for detecting the locations of arrows passing through a pair of adjacent penetrable targets, an optical housing disposed between the targets, a source of A highly collimated beam of light in the housing, a rotatable mirror in the housing disposed to sweep a beam of light from the source across each target as the mirror rotates, a reflective strip alongside each target opposed to the optical housing for receiving light swept across the target and reflecting the light back toward the rotatable mirror, a start photocell adjacent one end of each strip in the path of the light to denote the beginning of the sweep along each strip, an inclined mirror between the light source and the rotatable mirror for passing light from the source to the rotatable mirror and having a reflective surface to reflect light from the rotatable mirror reflected from each strip, and a stop photocell disposed to receive light from the inclined mirror and responsive to the change in light on passage of an arrow to denote the angular position of the arrow relative to the start photocell.
 10. In an apparatus for detecting the locations of arrows passing through a pair of adjacent penetrable targets, an optical housing disposed between the targets, a source of light in the housing, a rotatable mirror in the housing disposed to sweep a beam of light from the source across each target as the mirror rotates, a reflective strip alongside each target opposed to the optical housing for receiving light swept across the target and reflecting the light back toward the rotatable mirror, a start photocell adjacent one end of each strip in the path of the light to denote the beginning of the sweep along each strip, an inclined mirror between the light source and the rotatable mirror for passing light from the source to the rotatable mirror and having a reflected surface to reflect light from the rotatable mirror reflected from each strip, a stop photocell disposed to receive light from the inclined mirror and responsive to the change in light on passage of an arrow to denote the angular position of the arrow relative to the start photocell, a second source of light in the housing, a second rotatable mirror in the housing disposed to sweep a beam of light from the second source across each target on rotation, a second reflective strip alongside each target adjacent the first recited strip for receiving light from the second source and reflecting the light back to the second rotatable mirror, a start photocell adjacent one end of each second reflective strip in the path of light from the second source to denote the beginning of the sweep along the strip, an inclined mirror between the second light source and the second rotatable mirror for passing light from the source to the rotatable mirror and having a reflective surface to reflect light from the rotatable mirror reflected from each second strip, and a second stop photocell disposed to receive light from the second inclined mirror and responsive to change in light on passage of an arrow to denote the angular position of the arrow relative to the start photocell.
 11. A combination as defined in claim 5, wherein the rotatable mirrors each have two reflective surfaces to sweep two successive beams of light on each revolution.
 12. A combination as defined in claim 5, wherein the rotatable mirrors are disposed to sweep their beams in planes intersecting each other in the optical housing so that each mirror scans a forwardly disposed strip adjacent one target and a rearwardly disposed strip adjacent the other target.
 13. A combination as defined in claim 7, wherein said light sources comprise a laser unit for directing laser beams coaxially out opposite ends thereof toward the rotatable mirrors. 